Scientific Method —

Insert Star Trek cliche here

Quantum teleportation between an ensemble of atoms and light is achieved.

Quantum teleportation is one of those mysterious smoke and mirrors tricks which always captures headlines. Although it is called teleportation, no matter is shifted. Instead it is the quantum state of one object that is transferred to another object. This was first demonstrated using two light beams, in which the quantum state of one light beam was transferred to a second light beam. Next up, the teleportation of quantum states between single ions was achieved. Now, in an interesting development, physicists have managed to teleport the quantum state of a light beam to a large group of cesium atoms.

So, how do you teleport a quantum state? First you must create a pair of particles that are in an entangled state. These particles are then shared between the receiver and the sender. The sender takes the object to be teleported and the entangled particle and performs a joint measurement on them. The sender then tells the receiver the result of that measurement, who uses that information to perform a local operation on the entangled particle. These operations cause the object's quantum state to be transfered from the object to the receiver's entangled particle. Hence teleportation is achieved.*

This particular experiment involved using a magnetic field and a laser to put a trillion cesium atoms into a particular quantum state. These atoms are then entangled with a light pulse. Thus, the receiver is using a group of atoms as an entangled particle while the sender is using photons as the entangled particle. The object to be teleported is the quantum state of a second light beam. The measurement consists of combining the two beams at a partially reflective mirror and measuring their combined intensity with a light detector. The result of this measurement is then used to drive a radio frequency magnetic field to manipulate the quantum state of the cesium atoms. This replicates the polarization state of the second light field onto the spin state of the cesium atoms. At this point teleportation has occurred.

I know I shouldn't describe these experiments as smoke and mirrors but the name teleportation is so far from the popular culture definition that I can't help feeling a bit betrayed. The important thing to note is that the quantum state is transferred, not a measurement of that state. This means that the state could be a superposition state such as those that make up qubits. This is something that cannot be done with classical "measure and manufacture" experiments. That makes these experiments part and parcel of the development of quantum computing since manipulating,storing, copying, and transferring quantum states accurately and efficiently are a prerequisite to developing a useful device.

*Note to Star trek fans: do not try this at home. The fidelity of quantum teleportation experiments is theoretically limited to less than one, which means that you will probably be carefully transferred to your hodgepodge of specially prepared caesium atoms minus the states that make up your brain.

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku